During MR imaging (MRI), a uniform magnetic field is applied to an imaging region in which a subject is disposed. A set of gradient coils modulate this main magnetic field so as to provide spatially localized information, enabling the subsequent spatial encoding of signals. Radiofrequency coils generate and apply one or more radio frequency pulses to the subject in the imaging regions in order to alter the atom's magnetization, the energy which is subsequently spatially encoded and re-emitted in a manner which is characteristic of the nuclear magnetic resonance properties of the tissue of interest.
Each MRI scan is driven by a pulse sequence, i.e. a sequence of precisely timed events, including the generation of particular gradient field(s) and radiofrequency pulse(s), which coordinate and control the various system components and result in the generation of the above-noted signal and acquisition of data. These controls include the flow of current in the gradient coils and radiofrequency coils. Such gradient control signals are typically termed “gradient waveforms”.
Typically, gradient waveform design methodologies respect various MRI system constraints, such as maximum available gradient strength, duty cycles, nerve stimulation thresholds, and rise times (or gradient slew rates) per gradient axis. Gradient waveforms are often designed to serve a specific purpose when applied during MR imaging: to support slice selection, to encode k-space, or to spoil residual magnetization etc.
Driving of the gradient coils via these control waveforms usually creates various side effects that can negatively affect the image quality and/or MRI system stability. First, gradient waveforms that contain high frequency content can become distorted, since the gradient system (i.e. the amplifier and the coils) are inherently band limited (or low-pass) in nature. This distortion can lead to image artifacts. Secondly, changing the current in the gradient coils results in forces and torques on the gradient windings and the gradient coil, leading to vibration and noise.
This, and other similar vibrations, may reduce image quality, or lead to mechanical failures in extreme/repeated cases. Such vibrations also typically produce acoustic noise, like the “knocking” sound, which directly impacts on patient comfort. Since mechanical and acoustic resonances exist in any MRI system, certain driving frequencies tend to result in greater vibrations and/or acoustic noise than others during imaging.
In addition to noise and vibration, any coupling between the gradient coils and the magnet primary windings can lead to heating of the superconducting structures, which can also exhibit sharp frequency resonance behaviors. As noted before, such distortion can lead to image artifacts.
Such resonance “errors” during imaging may be reduced by better waveform design and production.